A BRIEF GEOLOGICAL HISTORY OF THE
EL PASO-JUAREZ (ELP-J) REGION

In order to document the many opportunities for study around our area,
the following is a condensed excerpt from a layman's guide entitled Geologic
Field Trip of the El Paso-Juarez (ELP-J) Area and has been revised
by Dr. W.C. Cornell
The geological history of the El Paso-Juarez area is an integral part
of the history of the North American continent. Continental masses have
grown larger through time by the addition of new crustal material around
their margins. Although the oldest rocks in North America are 3.96 billion
years (BY) old, the oldest ones in the El Paso area are only 1.2 to 1.4
BY old, relatively late additions to the continent. Most of the continent's
older rocks occur in the Canadian Shield, the nucleus about which continental
growth took place. There is controversy about where the continent ended
-- near El Paso or to the south, near Chihuahua City. Much of the rest
of modern Mexico consists of smaller continental and oceanic sea floor
fragments added through time like pieces to a jigsaw.

The oldest rocks in the El Paso-Juarez area were deposited along the
shoreline of a shallow tropical sea. Oldest of these is the
Castner Formation
which is exposed in a number of places along the eastern slopes on the
east side of the Franklin Mountains. These rocks record deposition of lime-rich
muds. Later, they were metamorphosed into marble.

Overlying the Castner Marble is a thin basalt flow known as the Mundy
Breccia. It is, in turn, overlain by a thick sequence of quartz sands which
have been metamorphosed to the
Lanoria quartzite. Many features of the
Lanoria are similar to those seen in modern beach systems such as the Texas
Gulf near Padre Island. This quiet scene ended abruptly about 1.1 BY ago
by an extended period of geologic disruption and violence. Chief of the
destructive processes was magmatism -- intrusion of great volumes of molten
rock. Magmas were silicon-aluminum rich and produced six varieties of granitic
rocks, collectively called the
Red Bluff Granite. Magmas intruded into
the Castner, Mundy, and Lanoria formations, heated them, and sometimes
ripped chunks of these rocks from the magma chamber walls. These chunks
formed structures called xenoliths within the solid granitic rocks.
At some time, some of the granitic magma breached the earth's surface to
initiate a series of volcanic eruptions. These eruptions included pyroclastic
ash-flow tuffs (the sort of eruption that destroyed the ancient city of
Pompeii in 79 AD) as well as numerous lava flows. Collectively, these volcanic
units are called theThunderbird Group.

Following the Red Bluff/Thunderbird events, things quieted down and
erosion of the igneous rocks began. This erosion lasted until about 500
MY (million years) ago when the rising sea level gradually flooded the
El Paso-Juarez region. The first of the marine sediments deposited over
the erosion surface was sandy material that was lithified to form the Bliss
Sandstone. For the next 250 MY, this region was part of the North American
continental shelf, a low-lying region very close to sea level. As sea level
fluctuated, the area alternatively was inundated by the sea and marine
sediments were deposited. Later, as sea level fell, the area stood above
sea level as dry land. During the submerged episodes, remains of a variety
of marine organisms were buried in the sediment and formed the fossils
characteristic of these
Paleozoic rocks. Many examples of these fossils
are on display in the Strain hall of Paleontology in the El Paso Centennial
Museum.

At the end of the Paleozoic Era, the American southwest was uplifted
above sea level, a position it occupied during most of the succeeding geologic
interval, the Mesozoic Era. Rocks were being eroded and little deposition
to form Mesozoic sedimentary rocks occurred around El Paso until the middle
of the Cretaceous Period, in late Mesozoic time. In the Cretaceous, The
El Paso-Ju rez area was near the head of an arm of the Gulf of Mexico
called the Chihuahuan Embayment. Shallow water marine sediment was
again locally deposited. Today, these sedimentary rocks are visible along
the base of the of the western slope of the Franklin Mountain, around the
flanks of Cerro de Cristo Rey, and throughout the Sierra de Ju rez.
Numerous fossils of marine invertebrates are present, but no remains of
terrestrial dinosaurs have been found in them. Examples are on display
in the Strain hall of Paleontology.

Sea level fell (or the continent rose) as the Cretaceous period ended
65 MY ago, and a new chapter in El Paso's geologic history began. The Cenozoic
Era, as the interval from 65 MY ago to present is called, was a time of
major change in this region. Early in the Cenozoic, mountain-building forces
again were in action and bodies of molten magma moved into the crust. At this
time, however, none broke through to form volcanoes; rather, the magma
bodies cooled in the crust forming andesite plutons. Some have been exposed
by later erosion -- the Campus Pluton, the Cristo Rey Pluton, and a number
of other, smaller ones. Emplacement of the plutons took place 45 to 50
MY ago. Shortly before plutonism occurred, immense compressive force developed
to the southwest and as a result, great masses of Cretaceous limestone
were thrust from the southwest to the northeast as a result. These masses
constitute the rocks of the Sierra de Juarez. Some geologists believe that
initial uplift of the ancestral Franklin Mountains took place at about
the same time while others believe that the Franklins are a younger feature
which formed only a few million years ago.

In any case, the compressive mountain-building forces waned and the
region was geologically quiet until about 35 MY ago when a new system of
stresses began. These were extensional, or "pull-apart" stresses
and they generated many of the local features. Perhaps the most obvious
is the Rio Grande Rift, a feature produced as the continent is being pulled
part in this region. The rift begins in Colorado, near Leadville, and extends
southerly through New Mexico to El Paso and on into Mexico where it seems
to die out. One product of the tensional forces and rifting has been the
formation of numerous elongate basins, separated from one another by mountains.
Local examples include the Hueco Mountains, the Hueco Bolson (or basin),
the Franklin Mountains, and the Mesilla Bolson. As they were uplifted,
the mountains began to accumulate in the basins, a process that continues
to this day. In addition to these sediments, other sediment was carried
into this area by the ancestral Rio Grande which, for a few million years,
emptied into Lake Cabesa de Vaca, a large, intermittent lake that occupied
the sites of the present-day basins. River sediment and lake sediments
are also part of the basin fill. Desert sand, blown into the area from
the southwest, is another constituent of the loose, unconsolidated sediment
o the basins. These sediments are called the Fort Hancock Formation. The
basins are deep structures -- more than 9,000 feet of sediment underlies
the El Paso International Airport and 12,000 feet of basin fill sediment
rests on Cretaceous rock under the Mesilla Bolson surface.

Associated with continental rifting, basaltic volcanic activity took
place. The basaltic magma arose deep in the crust or at the top of the
mantle, rose to the surface, and erupted to form the cinder cone volcanoes
and lava flows that dot the Mesilla Bolson surface.

During deposition of the Fort Hancock sediments, a diverse mammalian
community lived here. The skeletons (fossils) of creatures such as wooly
mammoth, giant ground sloths, horses, camels, and smaller animals occur
in the sediments. They are not common, but examples are on display in the
Centennial Museum, as part of a photographic exhibit of excavation of mammoth
bones from a sand quarry near the present site of Vista Hills Hospital.

The original Geologic Field Trip of the El Paso-Juarez
Area was written by Dr. P. C. Goodell, the Student Chapters of the
Society of Mining Engineers of AIME, the Society of Economic Geologists,
and the Lloyd A. Nelson Geological Society.